U.S. Pat. No. 4,652,115 to Palm et al. issued Mar. 24, 1987, which is assigned to the assignee of the present invention, discloses what the inventors believe to be the first practical full color double transfer electrophotographic print engine employing flexible belts. Said U.S. Pat. No. 4,652,115 is hereby incorporated by reference exactly as if set forth in full herein. The machine in U.S. Pat. No. 4,652,115 discloses a precision servo mechanism for controlling separated image registration as the composite image is built up on the transfer belt. In the print engine of U.S. Pat. No. 4,652,115, as in the preferred embodiment of the present invention, the photoreceptor belt is selected to be of a length which is nominally two times the length of the transfer belt. Under circumstances in which there is a precise integer relationship between the lengths of the photoreceptor and transfer belt, no problems in registration arise so long as the belt speed control is uniform and sufficiently accurate for both belts. Naturally, the conventional wisdom of the prior art, prior to the invention shown in U.S. Ser. No. 4,652,115, was that variations in belt length made full color copiers employing flexible belts impractical since a precise integer relationship between the belt lengths can only be defined for a given set of age and temperature conditions. Since it is preferable to have somewhat different mechanical/chemical characteristics between the photoreceptor and transfer belts, they tend to have different coefficients of thermal expansion, and also tend to stretch at different rates due to aging, and the differing forces applied to the respective belts.
Therefore, U.S. Pat. No. 4,652,115 shows the first practical precise belt control mechanism for achieving accurate registration of a plurality of separated color component images and a process for overlaying same to create a full color composite image which is ultimately transferred to a final image receptor.
In the apparatus of Palm's patent, all electromechanical elements in the image path are slaved to the speed and position of the photoreceptor belt. Palm shows apparatus which made adjustments, on each rotation of the photoreceptor belt, to a servo defining target positions for predetermined points on the respective belts such that the same points on the belt would come in contact during each rotation. While the Palm apparatus represents a good and practical device, the entire arrangement can give rise to large error signal outputs from the servos controlling same. This leads to significant power consumption and a requirement for larger motors.
As disclosed in U.S. Pat. No. 4,652,115, significant electrostatic forces are often present at the transfer station or "nip" where the photoreceptor belt and transfer belt contact each other. Depending on how these forces are controlled, they can work to the advantage or disadvantage of any given synchronization and registration scheme. The main parameters which control force at the nip are the applied and latent electrostatic voltages present on the respective belts, the physical area of contact, normally defined by the "wrap" at the transfer station and the amount of toner present in the nip.
In particular, the inventors of the present invention have studied problems of attractive forces between belts in a double transfer electrophotographic print engine. It has been found that the presence and absence of toner at a wrapped transfer station causes the total attractive force between the belts to vary by two orders of magnitude. It is believed that the toner acts essentially as a lubricant although its effect as a dielectric may also be significant.
Of particular importance to the present invention is the discovery that even a background amount of toner in an image area at the transfer station of the preferred embodiment causes a significant reduction in attractive forces between the photoreceptor and transfer belt. In this context, a background amount of toner is the very slight amounts of toner which are deposited on an image area of the photoreceptor belt passing under the decorator roller of a development station for an uncharged belt area. In other words, it is the residual toner deposited on a portion of the photoreceptor belt which should appear as white (or at least one having a complete absence of the pigment being developed on a given separated image) in the final printed image.
When a typical transfer station transfer corona is activated and a charged area of the photoreceptor belt is passing through the transfer station, the attractive forces between the belts are extremely strong and same are essentially locked into synchronism so that their contacting surface areas must travel at the same linear velocity through the transfer station.
Therefore, in keeping the transfer corona on constantly except when the seam areas were in contact was an element of the registration control apparatus of Palm's U.S. Pat. No. 4,652,115.
Naturally, the above described condition of charge and undecorated PC belt will occur normally in the inter image areas of the PC belt between image areas of successive latent and developed separated images. Since such machines conventionally include a charging corona for applying a uniform surface charge condition to the PC belt prior to its entrance to an image exposure station, the portions of the belt which do not form part of the image area remain charged since they are not illuminated at the scanning station. Also, in order to keep machine contamination to a minimum, it is customary to deactivate all the toner modules as such non-image areas pass under same to prevent full development of non-image areas which causes waste of toner and contamination of the machine. Thus, during normal operation, these charged areas will arrive at the transfer station essentially free of toner and fully charged. If a transfer corona at the transfer station remains on as this passes through the transfer station, it is virtually impossible for the belts to slide past each other to any degree.
As is also disclosed in U.S. Pat. No. 4,652,115, many of the practical physical devices for implementing photoreceptor and transfer belts are best constructed by employing a seamed belts, i.e., ones which are fabricated as an elongated sheets and joined end to end at a seam to form closed belts. Naturally, the seam represents a significant discontinuity in the surface characteristics of the belt and attempts to develop electrostatic images on the photoreceptor seam, or transfer and retain them on the seam of the transfer belt, should be avoided. In general, a seam area is a species of a non-image developing area on a photoreceptor medium and the seam on a transfer belt may be considered a non-image receiving area.
It should be noted that in the specification the terminology "image receptor", unless otherwise noted or improper from the context, is used to define an ultimate print receptor such as a sheet of paper, sheet of transparent material, or the like. An image receiving web refers to a web for receiving developed images comprising patterns of toner materials and also includes image receptors as a class. Therefore, image receiving web may be considered as a generic term which includes transfer belts and any belt or web material carrying an image receptor. The image receptor is the physical material which carriers the ultimate output of the electrophotographic print engine.
One of the difficulties encountered in the control apparatus disclosed in Palm'patent arises from the integer multiple relationship between the respective belt lengths wherein said integer is greater than one. In the preferred embodiment disclosed in Palm, as in the preferred embodiment of the present invention, the referenced integer is two since the photoreceptor belt is nominally twice the length of the transfer belt. In a position synchronizing registration control scheme such as that disclosed in Palm, there is only one opportunity (at the alignment of the seam areas) each two revolutions of the transfer belt in which to make an adjustment if there is a broad area of the nip between the transfer and photoreceptor belts.
U.S. Pat. No. 4,330,196 to Yamaguchi shows a desk top single transfer monochromatic electrophotographic print engine in which four small DC motors are used as prime movers in the machine. The motor on the decorator roller (rotating sleeve) of the development station is operated independently of the other three which need to be substantially synchronized. The other three motors drive the photoreceptor drum, the scanner, and the fuser mechanism. There are no substantial problems with registration presented in the type of print engine disclosed in Yamaguchi since the only registration problem to be solved in such a machine is accurate paper picking so that the leading edge of the paper or other image receptor will register with the leading edge of the developed image area.
Therefore, there is a need in the art for a photoreceptor/image receiving web synchronization arrangement for a full color electrophotographic print engine wherein photoreceptor and image receiving web may be driven at slightly different speeds, but which will still accomplish the accurate registration for a full color electrophotographic printing of the control apparatus disclosed in the Palm's patent.